An electrolytic capacitor that utilizes a metal with valve action such as tantalum, aluminum, or the like is generally widely used, because compact size and large capacity can be obtained by making the valve action metal as an anode side counter electrode into a shape of a sintered body, etched foil, or the like to enlarge a surface of a dielectric. In particular, a solid electrolytic capacitor in which a solid electrolyte is used as an electrolyte has compact size, large capacity, and low equivalent series resistance, as well as characteristics such as good processability into a chip and suitability for surface mounting, so the solid electrolytic capacitor is essential for making an electronic instrument with more compact size, higher function, and lower cost.
For small-size and large-capacity application, this type of solid electrolytic capacitor generally has sealed structure manufactured through a step of forming a capacitor element by winding an anode foil and a cathode foil composed of the valve action metal such as aluminum with a separator being interposed, a step of impregnating the capacitor element with a driving electrolytic solution, and a step of accommodating the capacitor element in a case made of metal such as aluminum or a case made of synthetic resin. Aluminum as well as tantalum, niobium, titanium, and the like are used as an anode material, and the same type of metal as the anode material is used as a cathode material.
Furthermore, although manganese dioxide and a 7,7,8,8-tetracyanoquinodimethane (TCNQ) complex are known as a solid electrolyte used for the solid electrolytic capacitor, a technology focusing on a conductive polymer such as polyethylenedioxythiophene (hereinafter referred to as PEDOT) which has a low reaction rate and excellent adhesion with an oxide film layer of an anode electrode exists in recent years (see JP H2-15611 A).
A solid electrolytic capacitor of a type in which a solid electrolyte layer composed of the conductive polymer such as PEDOT is formed in such wound-type capacitor element is fabricated as follows. First, a surface of an anode foil made of a valve action metal such as aluminum is roughened by electrochemical etching treatment in an aqueous chloride solution to form numerous etching pits, and then a voltage is applied in an aqueous solution such as ammonium borate to form an oxide film layer which acts as a dielectric (chemical conversion). Similarly to the anode foil, a cathode foil is also made of a valve action metal such as aluminum, and etching treatment is applied on a surface thereof.
The anode foil with the oxide film layer thus formed on the surface and the cathode foil are wound with an interposed separator to form a capacitor element. Subsequently, after a repair chemical conversion to the capacitor element, the capacitor element is discharged with a polymerizable monomer such as 3,4-ethylenedioxythiophene (hereinafter referred to as EDOT) and an oxidizer solution respectively, or immersed in a mixed solution of the both, to promote a polymerization reaction inside the capacitor element and generate a solid electrolyte layer composed of a conductive polymer such as PEDOT. After that, the capacitor element is accommodated in an exterior case with a closed-end cylindrical shape to fabricate a solid electrolytic capacitor.
Furthermore, the conductive polymer containing polypyrrole or polyaniline and an electrolytic solution containing γ-butyrolactone or ethylene glycol have been used in combination to reduce leakage current and improve ESR by action of the electrolytic solution to repair a defective part of a chemical conversion coating (see JP 11-186110 A).